1. Field of the Invention
The present invention relates to image processing of performing divided capturing of an object to generate image data representing the image of the whole object.
2. Description of the Related Art
To obtain a high-resolution image of an object, a system has received a great deal of attention which performs divided capturing of an object that is divided into a plurality of areas and joins the divided-captured images to generate an image having a very high resolution.
To join the divided-captured images, an overlap area where the captured images overlap each other is extracted, and the plurality of images are joined based on the overlap area. If the captured images include heterogeneity in such joining, no satisfactory joining result is obtained.
There roughly exist two kinds of “heterogeneity” included in the captured images: “heterogeneity of illumination” that occurs due to uneven illumination light for the object and “heterogeneity of vignetting” that makes the peripheral portion of a captured image darker than the center portion because of the lens characteristics. The heterogeneity of illumination readily occurs when the object size is particularly large. When images captured under illumination with large heterogeneity of illumination are joined, an image with large heterogeneity of luminance is obtained. The heterogeneity of vignetting occurs in each captured image. For this reason, a luminance difference is generated in each overlap area between the captured images, resulting in an unnatural joint.
Japanese Patent Laid-Open No. 2004-139219 (literature 1) proposes a method of correcting heterogeneity of vignetting of each captured image so as to obtain a natural joint between the images when joining a plurality of captured images. Although heterogeneity of illumination in a plurality of captured images is not corrected, Japanese Patent Laid-Open No. 2007-081580 (literature 2) proposes a method of capturing a color chart and a heterogeneity-of-illumination correction chart under the same illumination to remove the heterogeneity of illumination of the color chart, thereby calculating highly accurate color processing parameters.
In the method proposed in literature 1, the coefficients to be used to correct the heterogeneity of vignetting are expressed as a linear combination of known functions of image plane coordinates, and the linear combination constant is calculated by the statistical method. This method can correct heterogeneity of vignetting in each captured image to obtain a natural joint between the images but not heterogeneity of illumination remaining in the whole image after joining.
The method proposed in literature 2 corrects heterogeneity of illumination in an actual captured image using actual capturing data of the heterogeneity-of-illumination correction chart. However, since the method does not assume application to joining of divided-captured images, there is no description about correcting heterogeneity of illumination from a plurality of captured images of the heterogeneity-of-illumination correction chart. Hence, correcting heterogeneity of illumination when joining a plurality of captured images cannot be achieved only by the method proposed in literature 2.